The term “angiogenesis” mainly refers to a phenomenon in which newly formed blood vessels are formed from existing blood vessels. Examples of normal and physiological angiogenesis include angiogenesis in fetal life and angiogenesis involved in formation of the endometrium/corpus luteum, wound healing, or the like. On the other hand, it has been known that pathologic angiogenesis is deeply associated with the growth or metastasis of a solid tumor, chronic inflammation such as diabetic retinopathy or rheumatoid arthritis, and the like. In particular, tumoral angiogenesis has been vigorously studied from both sides of treatment and diagnosis. It has been considered that a tumor having a diameter of 1 to 2 mm can obtain oxygen or nutrient as a result of diffusion from existing blood vessels. However, for a further growth, angiogenesis is essential. In the case of healthy adults, since such an event as angiogenesis occurs only at a limited site in a limited situation, it is anticipated that a therapeutic agent or imaging agent targeting against tumoral angiogenesis can becomes a tumor-specific and universal agent and/or diagnostic agent.
At present, in the diagnosis of a tumor, PET (positron emission tomography) diagnosis using FDG (fluorodeoxyglucose) is carried out on a tumor site. However, such FDG only targets to a cell and/or tissue site having high glucose-metabolizing activity, and thus its tumor specificity is not sufficient. Since FDG highly accumulates even in the brain, heart, liver and the like as a result of physiological accumulation, it is problematic in that it may be difficult for FDG to carry out tumor diagnosis in some cases. In addition, in the urinary system such as the kidney, ureter or bladder, background increases as a result of a large amount of FDG discharged into urine, and thus it is difficult to make a diagnosis. Thus, targeting that is not mediated by sugar metabolism but mediated by another mechanism, and a targeting agent mainly targeting against angiogenesis has been developed.
On the other hand, application of an angiogenic event to treatments has been carried out as an angiogenic treatment. The importance of angiogenesis has been elucidated in wound healing or a therapeutic method for ischemic disease, or in treatments that are broadly called regenerative medicine, such as organ regeneration, cell transplantation, or the reinforcement of natural healing effects. Angiogenesis exhibits therapeutic effects by itself, or angiogenesis reinforces therapeutic effects. Accordingly, it is anticipated that a therapeutic agent, a targeting agent, and/or an imaging agent, which target against angiogenesis, can be used as an agent, a diagnostic agent, and/or a means for evaluating therapeutic effects in various treatments and regenerative medicines.
In particular, in the field of regenerative medicine, there have been a few methods for thoroughly analyzing therapeutic effects, and the authenticity of the therapeutic effects has only been indirectly evaluated by a combination of existing diagnostic methods. As described above, angiogenesis plays an important role particularly in regenerative medicine, but there are only a few methods of dividing newly formed blood vessels from existing blood vessels and evaluating them. It has been particularly strongly desired to develop an imaging means for visualizing only newly formed blood vessels. However, an imaging agent has various problems such as lack of specificity to newly formed blood vessels or lack of persistency, and consequently, sufficient results have not yet been obtained.
As a means for targeting against angiogenesis, a targeting agent and/or an imaging agent targeting to αVβ3 integrin, which is reported to be expressed at a high level in endothelial cells (and some tumor cells) during angiogenesis, has been under development. The αVβ3 integrin recognizes a peptide (RGD) consisting of a sequence of arginine-glycine-aspartic acid. Thus, on the basis of the RGD sequence, various circular RGD analog compounds or circular RGD-containing peptides have been particularly developed. For example, there are present many compounds such as cyclo-RGDfK, cyclo-RGDyV, cyclo-RGDfY and cyclo-RGDyK, which were produced from a circular pentapeptide, c-RGDfV, as a lead compound, which had been developed by Kessler et al. of Munchen Institute of Technology (Non Patent Document 1).
However, since the aforementioned circular RGD compound is rapidly discharged from the body mainly by renal excretion after it has been administered, its retention time in the body is short. Accordingly, when this compound is used as a targeting agent such as a drug delivery agent or an imaging agent, it is problematic in that a time in which its targeting ability can be utilized is short, and in that a majority of the compound is discharged from the body before it reaches a target site. Meanwhile, in the imaging of newly formed blood vessels, imaging and diagnosis, since a targeting agent is labeled with a probe such as a fluorescent dye or a radioisotope, it is required from the viewpoint of safety that a signal disappears as soon as possible after completion of detection and/or diagnosis of the target site, namely, that the targeting agent disappears from the target site at an early point after completion of diagnosis. However, even if the circular RGD peptide has reached a target site, it forms a strong bond with integrin which is expressed in newly formed blood vessels. Thus, the circular RGD peptide has been problematic in that it takes a long period of time until a signal disappears from a neovascular site in some cases. As a result, it has been desired to develop an imaging material, “the retention time of which in the body is long” and “in which a signal quickly disappears from the neovascular site.”
Meanwhile, biopolymers such as gelatin have been widely used as medical materials to date. However, it has not been known that such biopolymers can be used for the imaging of newly formed blood vessels. Along with the advancement of gene engineering techniques in recent years, protein synthesis by introduction of genes into Escherichia coli or yeast has been in progress. With the use of such techniques, various types of recombinant collagen-like proteins have been synthesized (e.g., Patent Documents 1 and 2). The synthetic proteins are non-infectious and thus superior to naturally occurring gelatins. In addition, the proteins are homogenous and have predetermined sequences and thus they can be precisely designed in terms of strength and degradability. Therefore, the use of such proteins is advantageous. However, in view of the use of recombinant gelatins suggested in the past, recombinant gelatins have been used as a replacement of naturally occurring gelatin. Needless to say, the use of recombinant gelatins as neovascular imaging agents has been unknown.